以Z-scan探討透明奈米懸浮粒子在甘油水溶液中平衡態下的非線性光學反應

Abstract

奈米懸浮溶液 (nanoparticle suspension) 非線性光學反應的研究已經有大約30年的歷史。 其中自聚焦 (self-focusing) 的研究可始於1982年A. Ashkin成功地在奈米懸浮液中利用光的梯度力場 (optical gradient force) 形成光孤子 (soliton). 而此種自聚焦的光也可利用Soret效應 (Soret effect) 在較低的入射光強度下形成。

這篇論文主要是利用Z-scan探討透明奈米懸浮粒子在甘油水溶液中平衡態下的非線性光學反應。在文章一開始，我們會先提出一個模型探討在光壓 (radiation pressure) 作用是否有可能在此懸浮液中形成自聚焦的光。在做了兩道雷射對打在懸浮液的實驗後，我們認為光壓不是主要的因素。但因實驗結果有圓對稱，因此我們假設由非線性效應引起的折射係數變化分布可視為是徑向距離的函數。為了了解這個分布，我們架設Z-scan的實驗並對不同樣本進行特徵曲線的測量。與理論得到的特徵曲線比較，我們認為觀察到的現象是一熱透鏡 (thermal lens) 加上Soret效應的組合。而其中也指出在溶液裡有奈米粒子的情況下，Soret效應是主要的非線性現象。最後，我們利用在此次實驗中學到的原理提出一種方法來得到單純由奈米粒子在溶液中造成的正規化穿透率變化 (normalized transmittance)。

The nonlinear optical response of the nanoparticle suspension has been a subject of interest for about three decades. For self-focusing, it can be dated back to $1982$ when A. Ashkin demonstrated the optical gradient force can give rise to self-trapped beams. Such self-focusing can also arises from the Soret effect, which can be achieved with much lower peak intensity when the particle size in the suspension is of the order of nanometer. In this thesis, we focus on the nonlinear optical response of the transparent nanoparticle suspension in glycerol solution using Z-scan technique. In the beginning, we propose a model to investigate the possibility of the self-focusing induced by the radiation pressure. A counter-propagating experiment shows the radiation force is not the dominant mechanism. But the experimental results possess radial symmetry. Therefore, we make the assumption that the refractive index change of the nonlinear optical response is a function of radial distance from the center of the laser beam. To investigate the form of this distribution, we set up a Z-scan experiment and obtain characteristic curves of various samples. Comparing these curves with that obtained from theoretical results, we suggest the dominant mechanism is a combination of the thermal lens and the Soret effect. Also, the results show that the Soret effect is the major nonlinear effect when there are nanoparticles in the solution. Finally, we discuss a way to extract the dynamics of the nanoparticle based on the principle of the Z-scan experiment.